How do we make our outputs to have the same size as the true mask?
You can create a mapping from classes to colors for a simple one is:
y # y.shape = (W, H, n_classes)
_, y_color = y.max(dim=-1, keepdim=True) / n_classes # y_color.shape = (W, H, 1)
y_color = torch.cat([y_color] * 3, dim=-1) # y_color.shape = (W, H, 3)


(using pytorch like code)
This mapping is visualizable, of course, you may get nicer visualizations if your class to color mapping is more sophisticated.